Sexually transmitted diseases (STDs) also known as Sexually transmitted infections (STIs) are diseases that are commonly transmitted between partners through some form of sexual activity, most commonly vaginal intercourse, oral sex, or anal sex. They were known as venereal diseases (VD) until some time around 1990, when public health officials introduced the new term in an effort to improve the clarity of their warnings to teenagers.

Note that allsexual behaviors that involve contact with another person or the bodily fluids of another person should be considered to contain some risk of transmission of sexually transmitted diseases. Most attention has focussed on controlling the HIV virus which causes AIDS, but each STD presents a different situation.

As may be noted from the name, sexually transmitted diseases are transmitted from one person to another by certain sexual activities rather than being actually caused by those sexual activities. Bacteria, fungi or viruses are still the causative agents. It is not possible to catch any sexually transmitted disease from a sexual activity with a person who is not carrying a disease; conversely a person who has an STD got it from contact (sexual or otherwise) with someone who had it, or their bodily fluids.

Although the likelihood of transmitting various diseases by various sexual activities varies a great deal, in general, all sexual activities should be considered as being a two-way route for the transmission of STDs (i.e., "giving" or "receiving" are both risky).

Health care professionals suggest safer sex, such as the use of condoms in any sexual activity, but safer sex should by no means be considered an absolute safeguard. Abstinence from sexual activities will protect against contracting a sexually transmitted disease.

Since prostitutes tend to have large numbers of sexual partners, prostitution has often been associated with the spread of sexually transmitted diseases. However, sexually transmitted diseases are transmitted by many other forms of sexual activity, so it is important that all members of the community use safer sex precautions, regardless of the nature of their relationships.

It is possible be an asymptomatic carrier of sexually transmitted diseases. In particular, sexually transmitted diseases in women often cause the serious condition of pelvic inflammatory disease.

The first venereal diseases clinic opened on January 31, 1747 at London Dock Hospital.